The use of embedded resistors inside of multilayer ceramic packages is well known in the art. Typically, resistors are formed when thin layers of resistive paste are deposited between dielectric sheets of green ceramic tape. After lamination and firing, the embedded resistors are then formed inside the multilayer package. Embedded resistors may be used for biasing, voltage dividing, and termination applications. A major problem associated with conventional screen-printing of resistive paste on ceramic substrates to form resistors is that there is an unacceptable amount of variability in the resistance values in a fired package. This is in part due to the fact that a single layer of resistive paste is merely about 0.3 mils thick and can vary substantially depending upon processing techniques. Screen printing processing variability may be caused by a variety of factors such as uneven fixturing, uneven squeegee blade or stroke, non-uniform viscosity of the paste composition, or other factors. As such, a fluctuation in the thickness of the printed film can result in a corresponding fluctuation in the amount of resistive paste deposited which necessarily effects resistance values in the resulting resistors.
It is not unusual to find resistance values that vary by as much as 30% from one multilayer ceramic package to another, although typically desirable resistance values may vary by about 10%. Obviously, this is unacceptable for any large-scale manufacturing process and also presents challenges to a designer designing the circuitry inside of multilayer packages.
Another problem with traditional resistor fabrication techniques is that the resistors require trimming as an additional processing step in order to achieve their desired resistance values. When resistors are embedded deep inside a multilayer ceramic package, they are not normally trimmable.
Consequently, many designs employ simple configurations in which the resistor is placed on a top layer so that it may be trimmed in an easily accessible manner. However, by placing resistors on the top layer, the top surface of the package leaves little room for other components such as integrated circuit packages or other components.
FIG. 1 shows a multilayer ceramic package with resistors in accordance with the prior art. Referring to FIG. 1, a multilayer assembly 100 is provided with vertically stacked sheets of a dielectric material 102 which are placed on top of a base substrate 101. Base substrate 101 may be either a single layer or a multilayer substrate package. Conductive layers 104 are positioned between the stacked sheets of dielectric material 102 to provide internal components. Resistors 106 are formed on a top surface 105 of the multilayer ceramic package. Significantly, the resistors 106 are formed from a very thin layer of resistive paste material, typically about 0.3 mils or less. Moreover, these resistors 106 are positioned so as to be easily accessible for a subsequent trimming operation.
A feature of a ceramic multilayer package that could reduce the resistance value variability to about ten percent or less while remaining susceptible to conventional multilayer processing techniques and which simultaneously created embedded co-firable resistors which did not require trimming to achieve their desired electrical characteristics would be considered an improvement in the art.